Typical designs for passive suspensions for seats comprise among other things, a guiding mechanism, a spring element as well as a damping element. The guiding mechanism has for example a scissor type mechanism, allow vertical movements of the seat, which are required during height adjustment as a function of the size of the operator, as well as movements caused by isolation of the suspension when the seat is subject to vibrations. The spring and damping elements allow an attenuation of the vibrations transmitted to the seat.
Pneumatic type passive suspensions typically integrate a pneumatic balloon as a spring element. This type of suspension generally has the disadvantage of offering suspension performances dependent on the chosen height setting. The dependence between the height adjustment of the seat and its suspension comes among other things from the dual role of the pneumatic balloon of the seat. Indeed, the driver can vary the volume of air in the balloon to adjust the height of the seat. Once the adjustment is made, the same balloon acts as a spring element for the suspension.
The first consequence of such a design is the geometric dependence between the height adjustment and the range of motion of the suspension. The effects of this can be observed notably when the height adjustment of the seat is a very high or very low position. In these situations, the end-of-travel abutments of the guiding mechanism of the seat interfere with the normal range of motion of the suspension by limiting its travel, which provokes frequent shocks that are uncomfortable for the driver. These shocks are harmful to the health and safety of the driver, and consequently opposite to the desired effect of a seat suspension, which consists to isolate the driver from any shock or vibration from the vehicle.
The second consequence of such a design is that, generally the stiffness of the balloon varies as a function of the height adjustment. Indeed, the seat occupant varies the volume air of the balloon to adjust the height of the seat. However, the more a balloon has a high volume of air, the more its stiffness is low, as long as the maximum volume is not met.
Furthermore, in the cases of most passive seat suspensions, the shock absorber fixed between the base of the seat and one of the scissor mechanisms has a rotational movement when the seat is in an ascending or descending motion. This non linear movement renders the rate of shock absorption non-constant over the complete range of height adjustment of the seat. Consequently, there is a third dependence between the performance of the seat suspension and the height adjustment.
A simple solution to decrease the level of incomfort of the shocks caused by the interference of the end-of-travel abutments mentioned hereinabove is the introduction of progressive end-of-travel abutments. However, in this case, the suspension looses a lot of its efficiency in the cases where the abutments often interfere with the normal range of motion of the suspension.
A better solution is a suspension, which is independent of the height adjustment mechanism of the seat. In such a case, the properties of the suspension, stiffness, range of motion and shock absorption, remain identical no matter what the height adjustment of the seat is, whether it is adjusted to its minimum or maximum positions.
The concept of a suspension independent from the height adjustment already exists. Patent applications CA 2,516,560 and CA 2,420,324 disclose the idea but apply it to an active suspension by specifying that the independence in the height adjustment is useful for limiting any drop of the seat caused by a break in the suspension or a failure in the actuator. Indeed, the drop is thus limited to the range of travel of the suspension and does not include the height adjustment range distance. These patent applications are not related to an improvement of the performance of compact passive suspensions.
Patent GB 2,309,894 also discloses an independent suspension from the height adjustment but only related to the range of motion of the suspension. Nothing else is disclosed, which renders the other suspension properties independent, including stiffness and shock absorption. Indeed, in this patent, since a suspension balloon is used with a volume of air that varies according to the height adjustment of the seat, this balloon suspension does not have consequently the same stiffness properties with high and low positions for the seat.
The objective of making the properties of the suspension independent from the height adjustment of the seat can be met by using two mechanisms that each fulfills its function without sharing common components. This can be accomplished by superposing a suspension mechanism stage on top of a height adjustment mechanism stage. Each mechanism stage has its own guiding systems, which allow vertical movement, in one case for height adjustment for the seat and in the other case for allowing a range motion for the suspension. The suspension stage has spring and damping elements that attenuate shocks and vibrations. The height adjustment stage has its own actuator in order to adjust the height as well as a locking system to fix this position. With this design, there are no common elements between the two systems, as opposed to what it seen commonly in pneumatic passive suspensions or with pneumatic balloons that play at the same time the role of a spring element for the suspension as well as an actuating element for adjustment of the height. Therefore, an independent suspension allows one to have a seat with properties of stiffness, dampening and potential range of motion that do not vary as a function of the height of the seat chosen by the operator.
However, this double mechanism requires a certain amount of space, which becomes a significant disadvantage in seat applications where certain drivers require a seating position which can be very low. Given that, for a very low driving position, the double mechanism would allow a normal range of motion of the suspension, which would cause movements that would be even lower than a normal driving position, their remains very little place between the bottom of the seat to introduce all the components of this double mechanism.
Thus, there is presently a need for a compact base for a seat with a suspension for a vehicle driver, in particularly for buses, for heavy vehicles or vehicles for special use, for example excavation vehicles or vehicles for forestry equipment, and in which the operator or the driver is subject to vibrations. Furthermore, there is a need for a compact base for a seat, which allows the suspension to be independent from the height adjustment for the seat. There is also to obtain total independence in the properties of the suspension, including stiffness, damping and range of motion, from height adjustment while offering a large range of motion for adjustment of the seat, as well as a very low minimum height adjustment position.